1. Field of the Invention
The present invention relates to a RF switch, and more particularly to a minute RF switch which can be used in a high frequency range from several MHz to several hundreds GHz.
2. Description of the Related Art
With the rapid progress of mobile telecommunication technology in recent years, the data rate that can be handled by mobile terminals has significantly been increased. Furthermore, to meet market demands for the higher telecommunication data rate, higher frequencies of signal career are being used so that mobile terminals can have wide bandwidth. At present, although mobile terminals use career frequencies ranging from several hundreds MHz to 2 GHz, they are expected, in the near future, to widely use higher career frequencies in the range of several GHz. In the field of wireless communications, high frequencies in a Ka bands from 20 to 30 GHz and a millimeter wave of about 60 GHz for vehicle communications have already been widely used.
FETs fabricated on a GaAs substrate are widely known as switches for handling such high-frequency signals. However, the FETs have a problem that they are expensive since they have to use GaAs substrate. Then, they cannot be constructed as large-scale components because they are expensive, making it difficult to integrate FETs with other devices. Another problem is that the higher frequencies of several GHz or higher tend to produce an increased energy loss, which fails to satisfy requirements for mobile terminals with low power consumption.
There are another known switches based on micro-electro-mechanical systems (MEMS). Since such switches can fabricate on any substrates, they can easily be integrated with other components. Furthermore, because they cause an extremely low energy loss, they are highly expected to be used in high-frequency applications. However, MEMS switches have a disadvantage that they are large in dimension, e.g., approximately size of 100 μm square, and need a high voltage of about 20 V to operate
As described above, the existing RF switches have disadvantages of their own. There has been a need for a new RF switch different from those existing devices. Generally, a switch is used to pass or block a signal flowing in a circuit by bringing about a large change in resistance or capacitance. OUM (Ovonic Unified Memory) developed by Intel utilizing the calcogenide semiconductor and PMC (Programmable Metallization Cell) invented by Axon are known as devices for causing large resistance changes.
The PMC disclosed in U.S. Pat. No. 5,761,115 will be described below. In U.S. Pat. No. 5,761,115, a device based on a phenomenon in which a metal dendrite is grown or retracted by a voltage applied thereto is referred to as a PMC, and the idea of using a PMC as a nonvolatile memory is described. Though it is not proposed to use a PMC as a RF switch in the description of U.S. Pat. No. 5,761,115, a PMC is interesting as a RF switch.
FIG. 1(a) of the accompanying drawings is a plan view of a PMC according to an embodiment disclosed in U.S. Pat. No. 5,761,115 and FIG. 1(b) of the accompanying drawings is a cross-sectional view taken along line A-A′ of FIG. 1(a). Lower electrode 93 is disposed over substrate 91 with insulating layer 98 interposed therebetween. Lower electrode 93 is patterned in a horizontal direction in FIG. 1(a). Second insulating layer 96 is disposed on lower electrode 93 and areas of insulating layer 98 where lower electrode 93 is not provided. Second insulating layer 96 has a via hole 99 defined therein which extends down to the surface of lower electrode 93. Fast ion conductor layer 92 is deposited on the inner side wall of via hole 99. Thereafter, the unfilled portion of via hole 99 is filled up with via filling layer 97. Upper electrode 94 is disposed on via hole 99. Upper electrode 94 is patterned in a vertical direction in FIG. 1(a).
When a voltage is applied between lower electrode 93 and upper electrode 94 with a negative voltage level on lower electrode 93, metal dendrite 95 grows from lower electrode 93 toward upper electrode 94 and finally reaches upper electrode 94. At this time, the electric resistance between upper electrode 94 and lower electrode 93 decreases. When the voltage polarity is reversed to apply a voltage between lower electrode 93 and upper electrode 94 with a positive voltage level on lower electrode 93, metal dendrite 95 is retracted from upper electrode 94 toward lower electrode 93. At this time, the electric resistance between upper electrode 94 and lower electrode 93 increases. U.S. Pat. No. 5,761,115 reveals an example in which the fast ion conductor layer is made of As2S3—Ag or a silver sulfide such as AgAsS2, the upper electrode (anode electrode) of silver or silver-aluminum alloy, and the lower electrode (cathode electrode) of aluminum. Interestingly, when the materials are combined as described above, the metal dendrite grows only when the voltage is applied between the lower electrode and the upper electrode with a negative voltage level on the lower electrode.
It has been found that some problems arise if the PMC disclosed in U.S. Pat. No. 5,761,115 is used as a RF switch.
The first problem is that the device is of a structure wherein two electric interconnects are connected to a switch, and a driver circuit for driving the switch is not isolate from a line for passing a data signal. To drive the switch, therefore, a signal has to be mixed with a data signal, posing a significant limitation on the design of the circuit.
The second problem occurs if the driver circuit is connected parallel to the line for passing the data signal in order to solve the first problem. In a high-frequency waveguide circuit, great care must be taken about an impedance change in the path along which the signal passes. The signal passing through the switch may leak to the driver circuit, thus allowing the switch to cause an increased loss. Depending on the impedance change, the signal may be reflected in the input port, and may not be transmitted in the output port.
The third problem develops if the driver circuit is connected to the signal line through an isolation circuit such as a transistor or the like in order to solve the second problem. In a low frequency range, it is possible to reduce the attenuation of the signal because the driver circuit is isolated from the signal line. At higher frequencies, however, a loss of the signal increases because the isolation characteristic of the transistor is degraded. The signal loss manifests itself at frequencies of several GHz or higher.
The fourth problem is that the whole switch is complex due to the need for a complex driver circuit. With the above intervening transistor, it is necessary to position the transistor as closely to the signal line as possible for the purpose of reducing reflections from the branch at the junction. However, sophisticated packaging technology is required to position the transistor as closely to the signal line as possible. An additional problem is that since the isolation device such as a transistor or the like is incorporated in the switch, the switch as a whole has increased dimensions, and the cost of the switch is high because an additional GaAs substrate is required to integrate the driver circuit.
As described above, even if conventional RF switches are improved using existing techniques, some problems remain unsolved.